1. Field of the Invention
The present invention relates to an improvement in a signal processing device for an automatic-focusing video signal in an electronically controlled photographic camera.
2. Related Prior Art
Electronically controlled photographic cameras have in general an automatically focusing optical system provided therein for detecting focus conditioning of the object to be photographed. FIG. 18 is a schematic diagram showing the arrangement of such automatic-focusing optical system, wherein 1 denotes a taking lens, 2 the object to be photographed, 3 a visual field mask, 4 condenser lens, 5 a diaphragm mask, 6 and 7 a separator lenses forming an image splitting optical element, and 8 a photosensor. The automatic-focusing optical system consisting of these elements is referenced 9.
In the automatic-focusing optical system 9, the visual field mask 3 is positioned near a film-equivalent plane 10, which plane 10 is in optically conjugate positional relationship with the object 2 through the taking lens 1; on the film-equivalent plane 10 there is formed a sharp image 11 of the object 2 when the taking lens 1 is suitably focused. The condenser lens 4 and the diaphragm mask 5 have the function of splitting the light passing through the taking lens 1 into two separate beams; the separator lenses are situated in a position which: is optically conjugate with the taking lens 1 through the condenser lens 4. The image 11 of the object formed on the film-equivalent plane 10 is reproduced by the separator lenses 6 and 7 as images 11' on two regions of an electric-charge accumulating part PD of the photosensor 8.
Assuming that the spacing between the suitably focused reproduced images 1 1 ' is l.sub.o, as shown in FIGS. 19 (a) and 20, the spacing between the images is narrower than l.sub.o when the taking lens 1 is positioned to focus before the plane 10 as shown in FIG. 19 (b), but the spacing between the reproduced images is wider than l.sub.o when the taking lens 1 is positioned to focus behind the plane 10. Since the change in the spacing between the reproduced images is proportional to the amount of defocusing of the taking lens 1, the spacing is measured and the data is processed by an AF-CPU(a central processing unit for automatic-focusing) so as to move the taking lens 1, depending on the direction and extent of defocusing of the lens 1, to thereby position the lens 1 at the suitable focusing position.
In order to perform such a process, it is necessary to process the AF video signal from the photosensor 8 in a predetermined manner. Electronically controlled cameras have a signal processor for processing the AF video signal for performing such predetermined process.
The signal processor for the AF video signal comprises: a self-scanning photosensor having a charge accumulating part for accumulating electric signal charge therein which corresponds to the luminance of the object, an exposure monitor for accumulating electric charge therein which corresponds to the electric signal charge accumulated in said electric charge accumulating part, and a dark-charge monitor for accumulating dark electric charge, the self-scanning photosensor being adapted to sequentially transmit the signal charge, which has been accumulated in said electric charge accumulating part, as the AF video signal; and a comparing circuit for setting a reference level by shifting the output level from the dark-charge monitor in such a manner that the potential of the electric signal charge accumulated in the charge accumulating part is driven to a predetermined level, and for providing, upon reversal of the reference level and the output level from the exposure monitor, an output signal which causes the AF-CPU to provide a command to cause the signal electric charge accumulated in the electric charge accumulating part to be transferred. In this signal processor for the AF video signal, the transfer command from the AF-CPU is used to drive a driving circuit for causing the signal electric charge accumulated in the electric charge accumulating part to be sequentially transferred as the AF video signal to an analog-to-digital (A/D) converter.
Meanwhile, a dark, low-contrast object will cause a relatively small amount of signal electric charge to be accumulated in the electric charge accumulating part, and will thus produce an AF video signal whose low-contrast corresponding level difference, which is the difference between high-luminance corresponding level thereof and the low-luminance corresponding level thereof, is relatively small. Thus, there is a disadvantage in that the AF-CPU cannot produce proper distance data using the AF video signal arising from such low-contrast object, since the repetition pattern in such AF video signal is weak.